(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to a driving circuit of a thin film transistor(TFT) LCD that compensates for the distortion of a common electrode voltage applied to a liquid crystal capacitor of each unit pixel in the LCD.
(b) Description of the Related Art
An LCD, which is one type of flat panel displays, uses the characteristic of the liquid crystal layers that can change the transmittance of light in accordance with a voltage applied thereto. An LCD has an advantage of being able to be driven at a lower voltage consuming little power. Such advantages have contributed to the widespread use of the LCD.
In an LCD, however, signal interferences between pixels deteriorates the display quality.
Such a deterioration of display quality in the conventional TFT-LCD will now be explained hereinafter with reference to the drawings.
FIG. 1 shows an equivalent circuit of a unit pixel in the conventional LCD.
As shown in FIG. 1, formed in the LCD are a liquid crystal capacitor Clc, which is formed by a pixel electrode 30, a common electrode 40 and liquid crystal injected between the pixel electrode 30 and a common electrode 40, and a TFT serving as switch that applies to the pixel electrode 30 a pixel voltage coming through a data line 10, controlled by the gate signal coming through a gate line 20. A storage capacitor may be formed in parallel with the liquid crystal capacitor Clc to improve the LCD""s ability to store charges.
Common electrode voltage Vcom applied to the common electrode 40 may be distorted because of the state of image signals applied to the LCD panel. The common electrode voltage is distorted mainly due to the following factors: a parasitic capacitor Cdc formed between the data line 10 and the common electrode 40; and the characteristic of liquid crystal display Clc of which capacitance changes in accordance with the magnitude of voltage applied to the liquid crystal.
Such a distortion of the common electrode voltage Vcom changes the magnitude of the voltage actually applied to both terminals of the liquid crystal capacitance Clc, the difference between the gray voltage and the common electrode voltage Vcom, thereby inducing a crosstalk phenomenon that deteriorates the display quality of contiguous pixels.
Such a crosstalk as caused by the distortion of the common electrode voltage develops also in the LCD employing a dot inversion driving method, which is intended to minimize the occurrence of crosstalk.
The distortion of the common electrode voltage in the LCD using the dot inversion driving method will be explained hereinafter with reference to drawings.
FIG. 2 shows a charged voltage of each unit pixel in the LCD using the dot inversion driving method.
FIG. 3 shows a schematic view of a gray voltage generating circuit in the conventional TFT-LCD driving circuit using a resistor string.
In the dot inversion driving method, a voltage of opposite polarity are applied to contiguous pixels of the LCD, and the voltage applied to the each unit pixel at each frame is the inverted voltage of the previous frame. A plus voltage is applied to the liquid crystal when applying gray voltage higher than the common electrode voltage to the liquid crystal capacitor Clc, and a minus voltage is applied to the liquid crystal when applying gray voltage lower than the common electrode voltage to the liquid crystal capacitor Clc. As shown in FIG. 2, voltages of opposite polarities are applied to the liquid crystal capacitor Clc of contiguous pixels. In more detail, as shown in FIG. 3, if the common electrode voltage Vcom of the gray voltage generating circuit is Va/2, the gray voltage lower than Va/2 (VG1, VG2) is applied to make the charged voltage in the liquid crystal capacitor minus and the gray voltage higher than Va/2 (VG3, VG4) is applied to make the charged voltage in the liquid crystal capacitor plus.
FIG. 4 shows a schematic view of the common electrode voltage and the gray voltage applied to the LCD when no crosstalk phenomenon occurs in the dot inversion driving method.
FIG. 5 shows the common electrode voltage and the data voltage applied to the LCD when the crosstalk phenomenon occurs in the dot inversion driving method.
As shown in FIG. 4, the gray voltage generating circuit in the LCD utilizing the dot inversion driving method applies a voltage lower than the common electrode voltage and a voltage higher than the common electrode voltage alternately to protect the liquid crystal layer against degrading, although the same color may be displayed on the LCD. Here, 1H in FIG. 4 refers to a period for which the gate line is turned on,
When the voltages charged in the liquid crystal in the horizontal direction of the LCD are (+), (xe2x88x92), (+), (xe2x88x92), . . . and displayed colors in the horizontal direction of the LCD are B(black), W(white), B, W . . . or W, B, W, B, . . . , the crosstalk phenomenon occurs.
In a TFT-LCD, an electric field caused by the voltage difference between the common electrode voltage Vcom and the gray voltage applied through the TFT is applied to the liquid crystal capacitor Clc, and the luminance and the transmittance of the liquid crystal layer are determined by the intensity of the electric field applied to the liquid crystal capacitor Clc. In the normally white mode LCD, when the difference of the voltage applied to both terminals of the liquid crystal capacitor is minimum, the LCD displays white, and when the difference of the voltage applied to both terminals of the liquid crystal capacitor is maximum, the LCD displays black. Accordingly, when white appears on the LCD, the amount of electric charges in the liquid crystal capacitor is minimum, and when black appears on the LCD, the amount of electric charges in the liquid crystal capacitor is maximum.
A drop of the common electrode voltage caused by the resistance changes in an electrode due to the difference in the amount of charges flowing through the common electrode, the voltage difference between both terminals of liquid crystal capacitor of horizontally contiguous pixels come to be different with the voltage difference between both terminals of liquid crystal capacitor of vertically contiguous pixels. This induces the distortion of a common electrode voltage waveform, as shown in FIG. 5. The gray level is displayed by applying charges proportional to an area C and an area D of FIG. 5. Then, because the difference between the area C and the area D is big, charges of the liquid crystal capacitor when a minus voltage is applied to the liquid crystal and charges of the liquid crystal capacitor when a plus voltage is applied to the liquid crystal become different, which degrades the precise display.
In other words, there occurs a difference in voltages applied to the liquid crystal although the same color is displayed on the LCD due to the distortion of the common electrode voltage waveform, causing crosstalk due to changes in the transmittance of the liquid crystal layer.
It is therefore an object of the present invention to prevent LCD""s crosstalk.
The LCD of the present invention comprises a liquid crystal display panel, a gate driver, a gray voltage generator, a source driver, a common electrode voltage generator, and a compensator for a distortion of common electrode voltage. A plurality of thin film transistors, a plurality of gate lines connected to gate electrodes of the thin film transistors, and a plurality of data lines connected to source lines of the thin film transistors are formed on the panel. The gate driver applies a gate signal through the gate lines of the panel and turns on and turns off the TFTs. The gray voltage generator generates gray voltages that have many voltage levels. The data driver applies to the data lines a data voltage for displaying picture signals on the liquid crystal display. The common electrode voltage generator generates a common electrode voltage applied to a common electrode of the panel. The compensator for a distortion of common electrode voltage is connected between the panel and the gray voltage generator and compensates for the distortion of common electrode voltage by changing the gray voltage.
The gray voltage generator comprises a plurality of resistors that is serially connected between a voltage source and a ground terminal, and divides the voltage of the voltage source, to generate a plurality of gray voltages of different voltage levels.
The compensator for a distortion of common electrode voltage comprises a capacitor that is connected between the common electrode of the liquid crystal display panel and the data voltage generator.
The compensator for a distortion of common electrode voltage may further comprise an amplifier that is connected between the common electrode of the panel and the capacitor, amplifying the distorted common electrode voltage to compensate for the distorted common electrode voltage measured at the common electrode of the liquid crystal capacitor.